For increasing the contact safety of electromagnetic relays, it is known, from German Patent No. 3,224,013, to provide the contact spring with a longitudinal slot to form two flexible ends and to equip each end with a contact piece which cooperates with a corresponding counter contact piece on a common fixed contact. The probability of both contact couples to fail due to contamination by minute glass fibers, moulding burs, or the like is substantially smaller than with single contacts.
These xe2x80x9ctwinxe2x80x9d contact springs, which are similarly known from German Published Application No. 1,175,807, German Utility Model No. 9,404,775, and German Patent No. 972,072, however, suffer from the difficulty that the spring arms formed by the longitudinal slot are more prone to breakage than the undivided spring. In such a case, while the relay per se is still operative, the broken spring arm may cause unpredictable short-circuits. A further problem of known twin contact springs resides in the fact that the individual spring arms are much softer than the undivided spring so that, when one contact becomes welded, the corresponding spring arm is not stiff enough to retain the actuator in the closed contact position. These properties prevent the use of known twin contact springs in safety relays.
German Patent No. 3,224,468 discloses a contact arrangement in which the contact spring carries two contact pieces each cooperating with a separate fixed contact. In addition to the fact that the total contact resistance of such bridge contacts is twice that of an individual contact couple, the known arrangement increases the safety in contact opening rather than in contact closure.
Further known are so-called xe2x80x9ccrownxe2x80x9d contacts in which at least one of two cooperating contact rivets has a raised peripheral portion which, if the two rivets are somewhat offset with respect to each other, form two contact locations. In addition to the fact that these contact locations have very small areas, the raised periphery is relatively quickly worn in use so that the intended double contact feature is rapidly lost.
It is the object of the invention to provide a contact unit which increases the reliability of contact closure in safety relays, i.e. relays with forcibly guided contacts.
To meet this object, a contact unit in accordance with the invention includes a fixed contact providing two contact locations and a contact spring having a longitudinal axis, a fixed end and a free end, and providing two contact locations which are disposed at an undivided portion of the free end, spaced in a direction transverse of the longitudinal axis and cooperating with the contact locations of the fixed contact, a zone of the contact spring between the fixed end and the contact locations providing increased torsional flexibility about the longitudinal axis.
The fact that the contact spring has at least one zone which exhibits increased torsional flexibility about its longitudinal axis ensures closure of the contact couples, which are constituted by the contact locations of the contact spring and the counter contact locations of the fixed contact, even when the free end of the contact spring is not parallel to the fixed contact.
In contrast to the twin contact spring referred to above, the contact spring of the contact unit according to the invention is undivided throughout its length, so that breakage is less likely to occur and, if it occurs, will cause the relay to fail completely by interruption.
Each of the two contact couples may be formed by at least two separate contact pieces provided on the contact spring and the fixed contact. Alternatively, the contact locations of the contact spring and/or those of the fixed contact may be formed at one common contact piece.
In a preferred embodiment, the zone of increased torsional flexibility may be formed by reducing the width and/or the thickness of the contact spring.
To achieve forcible guidance of the contact spring by the actuator, specifically during opening, an area of the contact spring extending from the location of engagement with the actuator to the contact locations may be stiffer than any other area of the contact spring. Increased stiffness may be achieved by increased thickness or by a deformed portion, preferably by a bead extending along the longitudinal axis throughout the length of the contact spring.
In order to use the torsional behavior of the contact spring effectively, it is of advantage for an actuator to have a convex portion for engagement with the contact spring, preferably a pair of convex portions engaging opposite sides of the spring.
In another preferred embodiment, the straight line connecting the two contact locations of the contact spring intersects at an acute angle the straight line connecting the contact locations of the fixed contact. An intentional inclination is thus provided between the contact spring and the fixed contact to form a first closing and last opening pre-contact and a last closing and first opening main contact. This arrangement has the advantage of softer contact closure with reduced bouncing.
The contact locations of the pre-contact may be made of a less noble contact material, preferably AgSnO, than those of the main contact, which are preferably made of an AuAg alloy. Additionally or alternatively, the contact locations of the pre-contact may be dimensioned larger than those of the main contact. The pre-contact is thereby provided with properties suitable for a load contact, while the main contact has the qualities of a signal contact.